The present invention concerns a process for the continuous production of lower (C.sub.1 to C.sub.4) alkyl esters of higher fatty acids by means of an essentially ambient pressure, catalytic transesterification process applied to an oil phase containing higher (C.sub.6 to C.sub.24) fatty acid triglycerides or natural oils or fats containing free fatty acids, and of C.sub.1 to C.sub.4 alcohols. The process is effected at reaction temperatures of up to 100.degree. C. in the presence of an alkaline catalyst, followed by separating glycerine, removing the catalyst residue and stripping off the lower alcohols.
This kind of continuous process is known, e.g. from FR-PS 2 560 210 or DE-OS 34 44 893 and in these methods, transesterifications of up to 95% and 97%, respectively, are obtained. Higher degrees of transesterification--about 98%--are obtained with a multi-stage process in accordance with FR-PS 2 577 569 or EP-B-O 127 104, which are carried out at pressures of up to 5 bars, in the presence of a liquid means of extraction. To obtain a higher output, the transesterification process can be carried out according to e.g. EP-A2-0 198 243 at higher temperatures of 210.degree. to 280.degree. C. and at 60 to 80 bars, with long reaction periods. In still other methods, e.g. in accordance with EP-B-10 192 035, ion exchangers are introduced as catalysts for pre-transesterification, or the ester phases are washed by means of cation exchangers, e.g. in accordance with OE-PS 386222.
All these methods have the disadvantage that they are not economical because they require the use of high temperature or pressure, a long reaction period or a high apparatus volume, or when a transesterification of up to 97.5 to 98.7% is obtained there is too large a portion of bound or, above all, free glycerine, and the acid value is too high, which requires expending a considerable amount of time and energy into separating the remaining glycerine and the resulting soaps and fatty acids, for example by means of an additional, final purification by distillation.
The suggestion was made in accordance with DE-PS 39 32 514 to introduce pipe reactors with additional static separators, whereby the reaction is carried out at very high rates of flow. This kind of method has become interesting of late because the fatty acid esters on their own, or together with methanol or gas oil, are suitable as fuel for diesel motors, as described in DE-OS 37 27 081 and further literature named therein.
The latest tests carried out by motor manufacturers show that it is important for the finished ester mixture which is used as high grade diesel fuel after transesterification to have a total glycerine content of under 0.20% of weight and preferably under 0.15% of weight. The free glycerine content should be under 0.01% by weight, the acid value should not be more than 0.2 and the triglyceride residue content should tend towards nil.
Up to now, these values could only be approximately obtained by means of a final purification by distillation, whereas the transesterification methods mentioned above, without a distillation step, just about meet the Austrian norm C 1190 which specifies a maximum of 0.25% total glycerine, 0.03% free glycerine and an acid value of 1.0.
All methods mentioned so far are based on the fact that esters suitable for use as diesel fuel are manufactured by transesterification (alcoholysis) of all sorts of vegetable fats and oils with lower aliphatic alcohols. For example, sunflowerseed oil, soya bean oil, corn oil, cottonseed oil, almond oil, groundnut oil, palm oil, coconut oil, linseed oil, and castor oil, and especially rapeseed oil, are considered appropriate. Transesterification is carried out with a suitable monovalent alcohol such as ethanol, isopropanol, butanol, or multivalent alcohols such as trimethylolpropane, but especially with methanol in the presence of a transesterification catalyst, e.g. metal alcoholoates, metal hydrides, metal carbonates, metal acetates or various acids, especially with sodium alkoxide or hydroxide or potassium hydroxide.
The object of the present invention is to provide a new method of the kind mentioned above, which can be carried out continuously and economically, whilst bringing about a transesterification of over 99.2 and up to 99.6%, and especially separating off the glycerine quickly and almost entirely, whilst avoiding to a large extent the formation of soaps, which would reduce the output.